1. Field of the Invention
The present invention relates generally to the prediction of weather disturbances and, more particularly, to the prediction of weather disturbances that give rise to microburst wind shear conditions at low altitudes over the earth's surface which are hazardous to aircraft during takeoff and landing.
2. Description of the Prior Art
Microburst wind shear is a weather condition which denotes significantly different wind velocities and directions occurring simultaneously at low altitudes over a relatively small region. A microburst wind shear typically lasts 5 to 15 minutes, occurs over a relatively small area, and is extremely hazardous during aircraft takeoffs and landings. Systems of the prior art generally detect microburst ground level wind shear after initial occurrence. In many landing and takeoff situations, these systems do not provide sufficient warning time to permit the avoidance of a wind shear area by aircraft taking off and landing, having provided the danger signal after the onset of the wind shear condition.
One method of the prior art for detecting surface wind shear conditions employs ground observations of wind direction and magnitude using mechanical wind sensors at a plurality of locations about an airport. This system has been proven to be inadequate since serious accidents have occurred at airports whereat such systems have been employed due to the untimely or missed detection of the wind shear conditions. A second method utilizes ground based radar. Ground based weather sensing radars typically have narrow antenna beams to enhance moist air detectability and provide high angular resolution. A Terminal Doppler Weather Radar, presently in development, is intended to detect surface microburst wind shear at an airport, as it develops, from a location about 20 Km from the airport. Because of geometric considerations, its doppler measurement capability is limited to detecting the horizontal movement of hydrometers (rain drops) above the earth's surface. It measures wind shear when there are a sufficient number of entrained rain drops to provide a detectable radar echo return. It can also measure horizontal movement of moisture laden winds aloft which provides indirect evidence of the presence of surface microburst wind shear precursors under some weather conditions. The probability of microburst precursor detection in this mode is not high and the false alarm rate, based only on these measurements, will likely be unacceptably high.
Other methods of the prior art utilize on-board apparatus for detecting the aircraft ground speed and comparing this ground speed to the airspeed of the aircraft. The difference in speeds and the vertical aircraft acceleration, determined by inertial sensors, provide an indication of the wind conditions about the aircraft. Such systems do not provide timely indications of wind conditions ahead and, in particular, do not provide advance warning of microburst wind shear ahead of the aircraft. Other prior art on-board wind shear detectors provide improved wind shear detection with utilization of data provided by the on-board vertical accelerometers, true airspeed indicator, pitch angle indicator, and angle of attack indicator to determine the rate of change of vertical wind and thus provide another wind shear indicator.
Systems which provide improved surface microburst wind shear detection with ground based equipment are disclosed in U.S. Pat. Nos. 4,649,388 (Re. No. 33,152) and 4,712,108. The invention disclosed in the former patent utilizes a doppler radar system with at least two vertically stacked radar beams which estimates the surface wind speed doppler spectrum of moisture laden air in each vertically stacked radar beam. If the horizontal wind velocity increases (or decreases) monotonically with altitude, the spectral components of wind velocity below (above) the point where the two spectra are equal are associated with wind velocities which occur below (above) the elevation angle where the two stacked beam patterns cross over. These wind speed spectral components provide an estimate of the radial doppler velocity resulting from horizontal wind shear as a function of range and azimuth, thus permitting the detection of the wind shear location and its magnitude. The invention of the latter patent provides surface microburst wind shear detection by processing horizontal doppler radar return signals of moisture laden air after the wind shear has occurred. By tracking the microburst wind shear center in range and azimuth as a function of time, the system determines the horizontal motion of the microburst wind shear centroid, thereby predicting the microburst wind shear location during its brief lifetime.
Though these systems may predict the future location of surface microburst wind shear by tracking a microburst after its initial occurrence, they do not have the capability, however, of predicting the initial microburst wind shear occurrence. Prediction of the future position of a microburst after its occurrence does not provide a warning of wind shear conditions to an airport at the initial microburst wind shear location. Another limitation of such doppler radar sensors is their inability to detect surface microburst wind shear in the presence of very little entrained moisture content, a phenomenon which occurs very often in the western part of the United States.
What is required is a system which reliably predicts the location of initial surface microburst wind shear with sufficient lead time for safely rerouting an aircraft about to land or delaying such landing and takeoffs of aircraft at that location and will perform this function even when there is little moisture in the entrained wind shear airflow.